1. Field of the Invention
The present invention relates to a process for the tubular blown film extrusion of a thermoplastic resin and more particularly and in a preferred embodiment, to an improvement in a process for the tubular blown film extrusion of a linear (low pressure) low or high density ethylene polymer.
2. Description of the Prior Art
In a conventional technique for forming tubular blown film suitable for the fabrication of bags and the like, a film-forming polymer, such as polyethylene, is extruded through an annular die arranged in an extrusion head so as to form a tube of molten polymer having a smaller outer diameter than the intended diameter of the eventually-produced film tube. In one technique, the film tube is drawn radially in its path upward from the die lips of the annular die by a force created by the differential pressure resulting from the cooling air flow from a venturi type lip air ring and the internal bubble pressure. The film tube is typically drawn radially only about one half to one inch prior to being contacted by the cooling air flow, and prior to contact it is usually drawn down to no more than half of its thickness at the die exit. The subsequent crystallization kinetics and rheological dynamics influence the resultant film optical and physical properties. Illustrative of prior art techniques utilizing the venturi type cooling modes and the effects upon film properties can be found for example in U.S. Pat. No. 3,167,814, 3,210,803 and 3,548,042. After cooling to solidify the molten tube, the tube is directed through flattening means such as a collapsing frame and a pair of driven rollers, to flatten the extruded film tube. Between the point of extrusion and the terminus of the flattening means, the film tube is expanded by means of air or some other gaseous medium to thereby form an expanded film tube and the film tube is maintained by the gas trapped within the expanded film tube between the die and collapsing means. The driven nip rolls draw the molten tubular film away from the annular die at a speed greater than the extrusion speed. This, together with the radial expansion of the molten film tube, decreases the film thickness and orients the blown film in both the machine and transverse directions. The degree of radial expansion and the speed of the driven nip rolls may be controlled to provide the desired film thickness and orientation. The location at which the film tube essentially completely solidifies is referred to in the art as the "Frost Line".
Thus, in short the polymeric material exits the die as a molten tube. It is subsequently expanded, drawndown and cooled and eventually becomes what is known in the art as a film bubble. The point of transition from a molten tube to a film bubble is not well defined, and hence for purposes of the present invention, reference will be made to a film tube to describe the polymeric material from its exit from the die to its final collapse at the nip roll.
Thermoplastic materials which may be formed into film by the tubular blown film process include polymers of olefins such as ethylene, propylene, and also include polyvinyl chloride, polystyrene, polyamide, polyesters, and the like. Of these polymers, low density polyethylene (i.e., ethylene polymers having a density of about 0.94 g/cc and lower) constitutes the majority of film formed by the tubular blown film process. As used herein, the term ethylene polymers includes ethylene homopolymers, and copolymers of ethylene with one or more comonomers. Conventionally, low density ethylene polymers have in the past been made commercially by the high pressure (i.e., at pressures of 15,000 psi and higher) polymerization of ethylene in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. Recently, low pressure processes for preparing low density ethylene polymers have been developed which have significant advantages as compared to the conventional high pressure process. One such low pressure process is disclosed in commonly-assigned, U.S. Pat. No. 4,302,565. It has also been recently determined that resins similar to the above low pressure process resins have been made in modified conventional LDPE equipment; e.g. tubular or stirred reactor equipment. Such resins have similar extensional viscosity indexes and the process of this invention will also apply to those resins.
The above-identified U.S. Patent discloses a low pressure, gas phase process for producing low density ethylene copolymers having a wide density range of about 0.91 to about 0.94 g/cc and a melt flow ratio of from about 22 to about 36 and which have a relatively low residual catalyst content and a relatively high bulk density. The process comprises copolymerizing ethylene with one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons in the presence of a high activity magnesium-titanium complex catalyst prepared under specific activation conditions with an organo aluminum compound and impregnated in a porous inert carrier material. The copolymers (as applied to these polymers, the term "copolymers" as used herein is meant to include polymers of ethylene with 1 or more comonomers) thus prepared are copolymers of predominantly (at least about 90 mole percent) ethylene and a minor portion (not more than 10 mole percent) of one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons which should not contain any branching on any of their carbon atoms which is closer than the fourth carbon atom. Examples of such alpha-olefin hydrocarbons are propylene, butene-1, hexene-1, 4-methyl pentene-1 and octene-1.
The tubular blown film extrusion process may be employed to form a film from low pressure-low density ethylene polymers. For example, a process for forming film from one such low pressure-low density ethylene polymer is disclosed in commonly-assigned, U.S. Pat. Nos. 4,243,619 and 4,294,746. However, it has been found that in some cases the film production rates obtained in tubular film processes with certain thermoplastic resins and particularly with low pressure-low density ethylene polymers, utilizing conventional cooling devices and techniques which cool with air rings of the type which direct air flow in a manner such as to create a reduced pressure zone e.g., by a venturi effect, are low. Many attempts have been made to increase tubular film production without sacrifice of film properties. Thus according to U.S. Pat. No. 3,568,252, there is disclosed a method of manufacturing a tubular film stably from thermoplastic resins. The method utilizes an annular cooling device having a coolant chamber provided with slits for blowing a gaseous coolant for preliminary cooling against a tubular film in a horizontal direction to no more than 30.degree. of angle of elevation. The device also includes means defining an inflating chamber for preliminarily inflating the tubular film thus preliminarily cooled and a second coolant chamber provided with slits for blowing a gaseous coolant for final cooling against the preliminarily inflated tubular film in parallel direction with respect to the running direction of the tubular film to no more than 30.degree. of inclination towards axis of the tubular film. The inflating chamber is interposed between the two coolant chambers and suction is created in the inflating chamber by the blown final cooling coolant.
Unfortunately, the process and apparatus disclosed in U.S. Pat. No. 3,568,252 is not entirely satisfactory for processing certain thermoplastic resins and particularly linear low density ethylene polymers because of the low rates obtained.
Specifically, the properties of low pressure-low density ethylene polymers are such that commercially desirable high film production rates have not been achieved without film tube instability. Stated conversely, film tube instability problems prevent the commercially desirable high film production rates from being obtained in blown film extrusion processes including those utilizing venturi action for cooling and film tube expansion with low pressure-low density ethylene polymers. Among the reasons for such failures, it is believed; is the extensional rheology of low pressure-low density ethylene polymers. When these low pressure-low density ethylene polymers are extruded from the die in tubular blown film processes and are externally cooled by blowing air against the resin with venturi type action as mentioned previously, the film tube becomes unstable by the increased cooling required by increased throughput rates. In other words, film bubble instability results at higher throughput rates since such rates require more heat transfer in the cooling process which is usually accomplished by increasing the amount and/or velocity of cooling air which in turn causes bubble instability e.g. the film bubble becomes non-uniform due to the extensional behavior of these low pressure-low density ethylene polymers.
Thus one of the major rate limiting factors in the extrusion of LLDPE blown film is reduced bubble stability due to the inherent low-strain hardening extensional behavior of the polymers. It would therefore be desirable to produce LLDPE blown film at high rates without sacrifice of film properties.